Each shaper tracks how far ahead the link has got with traffic that has been recently sent.
This depends on the length of packets sent and the speed of the shaper. This is, essentially, tracking how much is
likely to be queued at a bottleneck further on. The FB2500 does not delay sending packets and assumes something
with a lower speed is probably queuing them up later.
This record of how far ahead the traffic is gets used in two ways:
- If the shaper is too far ahead, then packets are dropped, causing the link to be rate limited
to the selected speed. Exactly how much is too far depends on the packet size, with
small packets (less than 1000 bytes) allowed more margin than large packets. This has the effect of prioritising
DNS, interactive traffic, VoIP, etc.
- Where there are two or more links with shapers a link is picked based on which is the least far ahead.
This has the effect of balancing the traffic levels between multiple links based on the speed of each link exactly.
